Catalytic membrane reactors using solid state membranes for facilitating various chemical reactions have been studied and used previously. For the membranes of the present invention, a valuable use of such catalytic reactors is in the oligomerization of methane to produce ethylene as a means to replace existing ethylene synthesis technology.
Currently in the U.S., approximately 75% of feedstock for ethylene production comes from natural gas liquids, primarily ethane and propane. The remaining 25% is obtained using naphtha derived from petroleum processing. Conventional technology is to "thermally crack," i.e. heat, the feedstock to produce ethylene. This produces ethylene, along with secondary reaction products. The cost of separating ethylene from the secondary reaction products can amount to 50% of the total process cost.
By contrast, catalytic reaction to achieve the same end result may utilize natural gas as the feedstock. Because of its very simple molecular structure, the use of natural gas as the feedstock minimizes the formation of secondary reaction products, thereby providing a clear cost advantage. However, current production of ethylene from natural gas using conventional catalytic reactors has a production efficiency of about 25%, whereas 40% efficiency is needed to achieve economic viability. In contrast to this, catalytic membrane reactors can achieve efficiencies close to 100%, with selectivity to ethylene also close to 100%.
All materials to be used in membrane reactors for the above-cited reactions must meet three requirements. First, they must be conductors of protons. The reactions of interest in this invention are based on the loss of hydrogen from the hydrogen-containing gas and therefore the membrane must be capable of mediating hydrogen away from the reaction surface. This hydrogen is converted into protons at the membrane surface and it is in the form of protons that the hydrogen is transferred through the membrane from the zone containing the hydrogen-containing gas to the zone with the oxygen-containing gas or vacuum. The second requirement for the membrane is the ability to conduct electrons. The reactions involving the loss of hydrogen from the hydrogen-containing gas, the transformation of this hydrogen into protons, and the subsequent recombination or reaction of these protons with the oxygen-containing gas are electrochemical and therefore generate or consume electrons. To maintain electrical balance, the electrons generated on the oxidizing surface of the membrane must migrate to the reducing surface on the other side of the membrane where they are consumed. Therefore, the membrane must be electrically conducting in order to allow for the transfer of electrons.
Finally, the third requirement for any materials to be used in membrane reactors is chemical and mechanical stability at the temperatures of operation and under the conditions of operation. Any reaction of the membrane material with the reactant gases leading to decomposition will cause the membrane to simply deteriorate with loss of proton and/or electron conductivity or failure of the membrane to completely separate the two reactant zones leading to direct reaction between the hydrogen-containing and oxygen-containing gases.
Perovskite materials have previously been used as membranes to facilitate the oligomerization of methane to ethylene (T. Hibino, S. Hamakawa and H. Iwahara, Chem. Lett., 1715 (1992) and P. H. Chiang, D. Eng and M. Stoukides, J. Electrochem Soc., 138, 611 (1991). Hibino and Chiang used the proton conductor SrCe.sub.0.95 Yb.sub.0.05 O.sub.3-x as the membrane material. Although this material is a proton conductor, it is not an electrical conductor and therefore does not meet the second requirement listed above. To use this particular material as the membrane in a catalytic membrane reactor, electrical current needs to flow through an external electrical circuit, complicating the reactor design. Also in both cases, product formation was minimal until a current was applied. This requires an energy input and adds expense to the cost of ethylene production. This is in contrast to the present invention in which electron conductivity is an inherent feature of the membrane and products are produced spontaneously, i.e. without added energy.
Two other types of membranes are known to achieve H.sub.2 transport. These are metal membranes and microporous membranes. An example of the former type is given in U.S. Pat. Nos. 4,388,479 and 3,393,098. This type of membrane suffers from the disadvantage of the expense associated with the metals which allow H.sub.2 diffusion. The microporous type membranes are fragile, difficult to fabricate, not completely selective to H.sub.2, and are not electrically conducting.
Among the metal type of membranes for H.sub.2 transport, earlier patents (U.S. Pat. Nos. 4,468,235 and 3,350,846) have addressed the problem of the high expense of these membranes by forming a multicomp membrane from a less expensive transition metal coated with the expensive H.sub.2 -transporting metal. However, this approach raises the problem of reaction between the two metals resulting in loss of H.sub.2 -transporting ability. Edlund et al. in U.S. Pat. Nos. 5,139,541 and 5,217,506 address this problem by interposing an inert proton-conducting material between the two metals. This serves as a buffer between the two metals to prevent reaction while still maintaining the H.sub.2 -transporting ability. Rather than using such a complex membrane, the current invention eliminates the use of the metal layers and forms a single component membrane directly from a single crystallographic phase electron- and proton-conducting oxide. This results in an inexpensive and rugged membrane.
In one of the above-cited applications, the membrane reactor will be used to decompose H.sub.2 S. For decomposition of H.sub.2 S, a membrane reactor has previously been described by D. Weaver and J. Winnick, J. Electrochem Soc., 138, 1626 (1991). In this reactor, the electrolyte serves to transport S anions, not protons. Although useful for this specific reaction, it clearly is not applicable to hydrocarbon dehydrogenation or oligomerization reactions.